Conventionally, raw sewage enters a wastewater treatment facility through an inlet pipe. An equalization tank or its equivalent may contain miscellaneous debris together with a sewage screen, grit chamber, and a waterwheel for transporting flow to a primary settling tank. The waterwheel may be driven by a shaft that is coupled to a rotating biological contactor (“RBC”).
The RBC typically has one or more banks of circular media (“disks”) that are supported by the shaft. Banks of disks are then rotated by the shaft, which may also be turned by an electric motor. As the media rotate into and out of the wastewater, microorganisms attach themselves to the media. Such microorganisms proliferate when exposed to, for example, ambient oxygen. Often, a system's capacity to treat wastewater is a function of the surface area of reaction products (“biomass”) thereby formed. As used herein, the term “biomass” means any organic source of energy that is renewable. Typically, contaminates contained in the wastewater include organic compounds such as hydrogen, carbon and oxygen that combine to form numerous other compounds. When this occurs over a large area, the weight of the dead microorganism is supported on the media surface area. Then an excessive build up will detach and fall. The microorganisms that consume the most common contaminates attach themselves to the media before multiplying to form a biomass in the form of a film.
Continuous rotation of disks in an RBC system provides the aeration and mixing necessary for treating wastewater. The effectiveness of the treatment and efficiency of output in part depends on oxygen transference. From the atmosphere, oxygen may be transferred by (1) absorption through a liquid film that flows over a disk surface; (2) direct oxygen transfer at an air/reactor liquid interface through turbulence created by rotating disks; and (3) direct oxygen absorption by microorganisms when exposed to the air.
A discussion of related technology is found in S. Cortez et al., “ROTATING BIOLOGICAL CONTACTORS: A REVIEW OF MAIN FACTORS AFFECTING PERFORMANCE”, Rev. Environ. Sci. Biotechnol. 7:155-172 (2008), which is incorporated herein by reference.
A flexible disk will not be straight during assembly and is unable to maintain a parallel relationship with adjacent disks. This creates an uneven distance between various surfaces, and microorganisms will be prone to bridge between pins or adjacent surfaces. This will cause the media bank or circular media to require a higher torque for rotation. This is undesirable because the system's useful life will be reduced.
In some cases, an RBC may have multiple stages (“media assemblies”).
Traditionally, in a first stage, the media surface area is presented to incoming wastewater, including microorganisms that naturally occur therein. As the disks rotate, the biofilm is formed on their surfaces. The biofilm metabolizes organic materials contained in the wastewater. In aerobic processes, rotating media promote oxygen transfer and maintain the biomass. Rotation also creates turbulence and enables excess solids to be removed or sloughed off from the media.
The microorganisms in typical wastewater normally form a microbial biofilm that grows over carrier material which is deployed in the disk bank(s) of the RBC. The media rotate at a speed of, for example, 1 to 5 RPM over a nearly stagnant bulk of wastewater and air. In operation, the film grows typically to a thickness of about 1-4 mm. When this occurs, the microorganisms that were the first to attach to the media die because no nutrient or oxygen can reach them. When this occurs over a large enough area, the biomass detaches itself from the media and sloughs off.
Media in use today typically include nested planar surfaces that are either flat or folded. They may, for example, be made of a mesh or a solid sheet. In known media assemblies, there is often a separation between adjacent disks. Extending from one side of conventional disks are projections that further engage the biomass. Both the flat and the folded types of media are spaced to provide a path that allows the sloughed biomass (i.e., biomass that has become detached from the media) to be removed from the media. This prevents plugging of the system which would reduce the media surface area exposed to contaminates in the wastewater and thus reduce the capacity of the system. Following traditional approaches, biomass interposed between adjacent disks may result in uneven spacing and the distortion of media surfaces. Such uneven spacing may produce clogging. In turn, this reduces the effectiveness of treatment and may weaken the disks that form the media.
Some media have variously shaped holes or open triangles that increase the surface area. However it is difficult using traditional methods to make a large section (e.g., 10 feet in diameter) and keep a flexible disk straight during assembly. Such components become expensive to produce, and generally have a shorter running life due to cracking and breaking.
In some prior art approaches, media growth of the biomass on some sections produces non uniform loading. This causes the media to exert a rotational torque that is higher than may be designed or desirable. Fracture of the shaft that carries the disk media often results and/or the media themselves may fracture. Moreover, prior art media often do not provide additional oxygen that is needed for biomass growth and survival to reduce sludge. Thus, prior art rotating media banks tend to collapse due to an imbalance of heavy buildup.
There are other disadvantages of conventional RBCs. From a process performance perspective, conventional RBCs have a limited oxygenation capacity. Because slowly rotating media may have about forty percent of their surface area immersed in the wastewater, a “dead zone” may exist near the center shaft. Solids accumulate near the center shaft because radial velocities are minimal, and the area is subject to little water scouring. Hence, traditional RBCs often fail to perform to the specifications of the original design. As noted above, the media have experienced problems of collapsing due to the weight of the solids build-up and retention. Additionally, many problems have been realized in the failure of the plastic media sheets to bond to the center shaft with which they rotate. Shaft problems have been experienced where the shaft shears under the load of excessive solids retained by the media. Because of this anticipated excessive loading, bearing problems have also been realized.
U.S. Pat. No. 7,156,986 is incorporated by reference. It discloses a pin and cone approach to spacing the media. However conical sections with long pins are difficult to manufacture economically. They are more rigid and can break (fail) during assembly. U.S. Pat. No. 7,879,232 discloses double-sided self-cleansing media. It is cumbersome to form a circular section. Cutting the periphery into a circular section can be challenging. That patent also is incorporated herein by reference.
It discloses a solid pin and solid cone approach to provide the equal spacing of media disk bank. However a solid conical section with a long pin tends to warp and a perpendicular relation is not maintained. This creates a mismatch and uneven gap between adjacent surfaces in the disk bank. Also, two media cannot easily be held together during assembly.
It would be desirable to augment the capacity of such treatment facilities without excessive cost and without impairing the effluent quality.